The above-mentioned motor pump employ two different capacitors, with one for starting the motors and the other for operation. Motors of this type (capacitor-run motors) have a good performance during both starting and running, and are usually used to drive a relatively big load (typically of 1.5 KW or more), while capacitor-start motors handle a relatively small load (typicall of 0.75 KW or less).
In the prior art, both the starting capacitor and running capacitor are located external to the motor-pump assembly (typically in a power distribution panel) because if they were built into the motor-pump assembly, this would require an impractically large space because of the physical sizes of the capacitors. Additionally, such a prior art arrangement usually employs an external trip switch in the form of a current relay or a timer which controls the starting capacitor. A switch of this type, however, does not always provide an optimal switching or tripping point at which the starting capacitor is released or disconnected from the associated winding of the motor, because it operates depending on parameters (i.e. current or time) other than those which are directly related to the actual torque required by the load and, therefore, the switching point is subject to variations due to fluctuations in supplied voltage. A premature disconnection of the start capacitor will not put the motor into operation but into "locked" state, resulting in possible damage thereto.
One way to get rid of this problem is to use a centrifugal switch which is mounted on the drive shaft of the motor and operates at a predetermined rotational speed of the motor for opening the starting capacitor. However, the use of such an internal switch requires an additional conductor to connect the internal switch with the external starting capacitor. This means that the cable connecting the panel and the motor assembly is of a five conductor type which is not readily available but normally needs to be ordered from a cable maker.